add macOS specific instructions

[miguelmota]

I was running into an openssl lib path issue when installing on macOS:

$ make
cc -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long    -c -o main.o main.c
main.c:11:10: fatal error: 'openssl/err.h' file not found
#include <openssl/err.h>
         ^~~~~~~~~~~~~~~
1 error generated.
make: *** [main.o] Error 1

then stumbled across this answer that mentions you have to export flags for specifying openssl lib paths:

export LDFLAGS="-L/usr/local/opt/openssl/lib -L/usr/local/lib -L/usr/local/opt/expat/lib"
export CFLAGS="-I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include"
export CPPFLAGS="-I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include"

and now make works:

$ make
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o main.o main.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o keys.o keys.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o hash.o hash.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o base58.o base58.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o result.o result.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o combination.o combination.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o applog.o applog.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o utility.o utility.c
cc -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include -D OS_UNIX -ansi -Wall  -O2 -Wno-long-long  -I/usr/local/opt/openssl/include/ -I/usr/local/include -I/usr/local/opt/expat/include  -c -o prefix.o prefix.c
cc -o bitcoin-tool main.o keys.o hash.o base58.o result.o combination.o applog.o utility.o prefix.o -L /usr/lib -lcrypto -lssl

[streamofstars]

In my case it was not enough, I had to also do:
export LIBRARY_PATH="/usr/local/opt/openssl/lib"

[Velua]

Legend! Thanks this was helpful.